The compression ratio of an internal combustion engine, broadly defined as the ratio of the maximum cylinder volume to the minimum cylinder volume, is an important parameter for controlling engine behavior. The compression ratio influences many factors, such as torque, fuel efficiency, emissions, cylinder pressures and temperatures.
Some internal combustion engines have a fixed compression ratio, selected to provide an acceptable trade-off of performance parameters. For example, for a diesel engine, the compression ratio is high enough to ensure compression ignition at cold ambient temperatures, without resulting in excessively high cylinder pressures at full load.
Engines having a variable compression ratio (VCR) have a means of controlling the compression ratio so that improved trade-offs can be realized. For example, a variable compression ratio might provide a higher compression ratio for starting the engine and a lower compression ratio at full load operation.
One approach to providing a VCR engine is to provide controllable changes to the piston geometry, which influences the cylinder volume. In the past, such VCR pistons have reduced the range of maximum cylinder pressure experienced by a particular engine, the piston geometry changing so that cylinder pressure does not exceed a certain value, under most, but probably not all, circumstances. The piston geometry changes are usually achieved over several engine cycles. Depending on engine conditions, the number of cycles for the compression ratio to change, by five ratios for example, may be from 20-30 cycles.